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My vent stack on my house keeps freezing completely blocking with ice at the top. Results in trap getting drained and sewer smell in the house. Why would the vent get ice? It is not covered with snow. Is there any way to prevent this. Resolved it by climbing up and pouring hot water on but 2 weeks later froze again.

As water enters into the drain system, especially warm or hot water, it condenses. This condensation vapor then moves upwards, being lighter than air, through the venting system. As it reaches the cold pipe and air at or near the roof line, it then begins to reform into droplets and freeze on contact with the cold pipe. With enough cold temps and time, it will ice over.
Do you have access to the vent pipe where it exits the roof? If so, you could wrap the vent pipe with insulation. This will prolong the ice-over process, not eliminate it but worth a try. Once the winter is over, you could consider enlarging the vent pipe by a diameter or two right below the roof line (in the attic).

You may find as stated,insulating the stack in the section from the ceiling to the roof helps and or cures your problem.But if not over sizing the vent terminal,just before it penetrates the roof is the approved frost proof termination of the vent.Good Luck.

1. Paint the vent pipe black so it absorbs a little heat from the sun.
2. Try to have the vent pipe come out on the sunny side.
3. Increase the size of the vent pipe...@-30, freezing occurs 1"per 12 hours.
4. For long periods of cold weather install a temporary heat tape down the vent pipe on the roof. You only need about a 1" opening for a vent in these very cold times to protect trap seals.
5. Having two or more vents penetrating the roof will also help with temporary frost closures in the future.

"The society which scorns excellence in Plumbing as a humble activity and tolerates shoddiness in philosophy because it is an
exalted activity will have neither good Plumbing nor good philosophy: neither its pipes nor its theories will hold water." -
John William Gardner 10/8/1912 - 2/16/2002

>from a point 1' beneath the roof, to 1' above the roof<
then ,pray tell, what do you from a point 1' above the roof for the next 6" of pipe?
mass code states the minimum height thru the roof is 18". (sal says so)
[news.bostonherald.com]

I believe it is the Venturi effect that from where the drastic change in temperature occurs.So from one inch below the roof connection to the pipe to it's terminal 18" above would still be correct.The throttling occurring at the meeting of the piping with the cold roof would be the area of change.Much the same as the introduction of air through positive pressure.jacuzzi of freeze.Same positive, draw.

No it has nothing to do with Venturi. The increased size under the roof would normally be at a higher temperature than the outside air so its greater volume would help heat the outside portion, and the increased size would require more frost to close it. It would also be cooler than the portion of piping in the building so it would possibly condense some of the water out of the air before it reached the upper portion where some of the remaining moisture would freeze.

What I find mostly happening is when we get enough snow to cover the roof vent/s closing shut the roof vent can occur at temperatures even above zero.

What happens is water vapor rises in the vent and comes in contact with the snow forming an icy barrier mix to block off the vent, I then just use a broom handle to breakup the obstruction

Keep in mind snow just by itself will not impede the function of the plumbing vent.

Of course the frost closure becomes more problematic when the weather gets colder, more snow on the roof, and larger amounts of hot water used in the household. I have not noted any hard and fast rules for vent frost closure as we have for water pipe sizing or DWV sizing other then what I have stated.

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